BQ76920EVM: Operating EVM with 3s40p configuration of 18650 cell battery pack

Hi Swaroop,

Cells are typically paralleled either to gain more capacity (run time) at the same current, or higher current.  Both come together although the physical interconnection of the cells will have some resistance and will limit the current which can be delivered.  As an example, if each cell could deliver 100A when shorted, 40 in parallel could theoretically deliver 4000A.  Again the physical connection of the cells will likely reduce that but the current delivered is likely to be very large.  Be sure to access the suitability of the test on your specific cells and situation.

1. The bq76920EVM board will operate with a 3s battery, however note the potential current comment above with the parallel cells.  The sense resistor, FETs, board connectors, and traces on the EVM all have limited current capability.  The board was rated at 15A (see the EVM user guide section 1.3), at higher currents the heating may be excessive.  The FETs used on the board show a 187A maximum pulse current. While the capacity in section 1.2 is shown as "Any", see the note at that section, it is expected that a 1 or 2P pack. While we would not recommend connecting the board to such a large capacity pack, if the current is limited to the board it may work. The current gauge revision is the bq78350-R1, if it is used, it will have a data range limit for capacity, see the bq78350-R1 documentation for how to scale the values.  The bq76920EVM does not include fuses, be sure to connect your pack with appropriate fuses. Other precautions may include planning for high temperatures and flying objects. Be sure to follow any safety regulations required for your facility or agency.

2. The bq76920EVM does include short circuit and overcurrent  discharge proteciton in hardware in the bq76920.  Settings are programmable, see the datasheet.  If the bq78350 gauge is used a lower overcurrent limit can be applied. The default hardware short circuit limit for the board would be 22A and overcurrent 8A, but these only work if the board has not been shorted or fused before the proteciton can operate.  

3. Since the battery described would seem to have more current capability than the board can support, if you decide to test be sure to limit the current, provide fuses, protect yourself and other users from any high temperature or flying objects resulting from the test, and follow any safety requirements or guidelines applicable in your labratory.

Thank you WM5295 for your detailed valuable reply.
I will consider all the precautions and measures mentioned by you during testing.

I want to test a 40p3s (40 cells in parallel and 3 such stacks in series giving a maximum total voltage of 12.6V) of 18650 cell configuration.
Considering each cell capacity to be 2Ah, so theoretically total current is 80A.

Q1. I want to discharge the 40p3s pack (each cell of 2Ah) at 0.3C to 0.5C rate and not beyond this, is it possible to have a control over the discharge rate of the battery pack in bq76920EVM or there are any other solutions?

Thanks and regards!

Hi Swaroop,
The capacity will be 40x 2Ah or 80Ah, and the 1C rate would be 80A, but the instantaneous current available will depend on the resistance of the cells, the interconnect, the board with its FETs and sense resistor, and the load resistance. If the load is a short circuit, the instantaneous current will be very large.
If the discharge rate is .3C or 0.5C maximum the current would be 24 to 40A, this is above the rating we placed on the board. The bq76920 will turn off the FET if its set thresholds are exceeded, and the thresholds are in the range to meet your current, but the board traces and components may not be large enough to sustain the current or dissipated power while the bq76920 is timing the duration of the current over the threshold without failure. You may be able to test at low currents to analyze where extra components are needed and build an alternate current path if needed for the full current capability of the pack.

Thanks again WM5295 !

As the bq76920EVM is rated at 15A, I shall select the C rate (for charging and discharging of the 40p3s configuration Li-ion battery pack) such that it is below the 15A in total current flowing out of the battery pack.

I shall operate the bq76920EVM at low currents, and will keep in mind regarding the instantaneous current.

Thanks a lot for your valuable suggestions.
regards